Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications, such as angina pectoris and incontinence. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. More pertinent to the present inventions described herein, Deep Brain Stimulation (DBS) has been applied therapeutically for well over a decade for the treatment of neurological disorders, including Parkinson's Disease, essential tremor, dystonia, and epilepsy, to name but a few. Further details discussing the treatment of diseases using DBS are disclosed in U.S. Pat. Nos. 6,845,267, 6,845,267, and 6,950,707, which are expressly incorporated herein by reference.
Each of these implantable neurostimulation systems typically includes one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator implanted remotely from the stimulation site, but coupled either directly to the neurostimulation lead(s) or indirectly to the neurostimulation lead(s) via a lead extension. A single stimulation lead may contain electrodes of different sizes. The neurostimulation system may further comprise a handheld external control device to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected electrical stimulation parameters.
Electrical stimulation energy may be delivered from the neurostimulator to the electrodes in the form of an electrical pulsed waveform. Thus, the stimulation energy may be controllably delivered to the electrodes to stimulate the tissue. The set of electrodes, including those on and off the lead, used to deliver the electrical pulses to the targeted tissue constitutes an electrode set, with the electrodes capable of being selectively programmed to act as anodes (positive), cathodes (negative), and/or left off (zero). In other words, an electrode set represents the polarity being positive, negative, or zero. Other parameters that may be controlled or varied include, but are not limited to, the amplitude, width, rate, regularity, and ramp of the electrical pulses provided through the electrode array. Each electrode set, along with its electrical pulse parameters, can be referred to as a “stimulation parameter set.”
With some neurostimulation systems, and in particular, those with independently controlled current and/or voltage sources, the distribution of the current to the electrodes (including the case of the neurostimulator, which may act as an electrode) may be varied such that the current is supplied via numerous different electrode configurations. In different configurations, the electrodes may provide current or voltage in different relative percentages of positive and negative current or voltage to create different electrical current distributions (i.e. fractionalized electrode sets).
As briefly discussed above, an external control device can be used to instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. Typically, the stimulation parameters programmed into the neurostimulator can be adjusted by the user by manipulating controls on the external user control device to modify the electrical stimulation provided by the neurostimulator system to the patient. Thus, in accordance with the stimulation parameters programmed by the external control device, electrical pulses can be delivered from the neurostimulator to the stimulation electrode(s) to stimulate, activate, or affect a volume of tissue in accordance with the set of stimulation parameters and provide the desired efficacious therapy to the patient. The best stimulus parameter set will typically be one that delivers stimulation energy to the volume of tissue that must be stimulated in order to provide the therapeutic benefit (e.g., treatment of pain), while minimizing the amount of non-target tissue that is stimulated. A typical stimulation parameter set may include the electrodes that acting as anodes or cathodes, as well as the amplitude, duration, and rate of the stimulation pulses.
To facilitate the selection of the stimulation parameters, the clinician generally programs the external control device, and if applicable the neurostimulator, through a computerized programming system. This programming system can be a self-contained hardware/software system, or can be defined predominately by software that is run on a standard personal computer (PC). The PC or custom hardware may actively control the characteristics of the electrical stimulation generated by the neurostimulator to allow the optimum stimulation parameters to be determined based on patient feedback, or other means, and to subsequently program the external control device with the optimum electrical stimulation parameters.
When electrical leads are implanted within the patient, the computerized programming system may be used to instruct the neurostimulator to apply electrical stimulation to test placement of the leads and/or electrodes, thereby assuring that the leads and/or electrodes are implanted in effective locations within the patient. Once the leads are correctly positioned, a fitting procedure, which may be referred to as a navigation session, may be performed using the computerized programming system to program the external control device, and if applicable the neurostimulator, with a set of stimulation parameters that best addresses the disorder or painful site.
Significantly, there are limits to how much charge (both in terms of total charge per pulse (or phase) and charge density per pulse) can be injected into tissue using one manner (e.g. biphasic, charge-balanced waveforms) without causing cell trauma and/or electrochemical damage (i.e., corrosion) to the electrodes. Each electrode, depending upon its physical properties (which include, but are not limited to, its size, shape, material, surface characteristics, and/or state), has a charge threshold level (which may also be affected by implant location, adjacent tissue type, and other biological factors) that should not be exceeded to ensure that the amount of charge applied to the electrode will not cause irreparable electrochemical harm to the electrode or induce cellular trauma. Smaller sized electrodes generally have lower charge threshold levels than larger sized electrodes that are manufactured of the same material because the smaller sized electrodes have higher charge densities.
Thus, with regard to tissue safety, both total charge and charge density have been taken into account to avoid cell trauma. As such, the Shannon model, which accounts for a single electrode of a surface area “A” through which a charge amount “Q” is injected, was created in 1992 for evaluating tissue safety limits. In particular, the Shannon model calculates a k-value in accordance with the equation:
                    k        =                                                            log                10                            ⁡                              (                                  Q                  A                                )                                      +                                          log                10                            ⁡                              (                Q                )                                              =                                                    log                10                            (                                                Q                  2                                A                            )                        .                                              [        1        ]            
(See Shannon, R. V., A Model of Safe Levels for Electrical Stimulation, IEEE-TBME, Vol. 39, No. 4, pp. 424-426, April 1992). It should be appreciated that the value of k comprises two terms: the log of the charge density, and the log of the charge. The author proposed that a tissue safety limit of k equal to 1.5 or lower should be maintained to ensure tissue safety given the assumptions listed in the publication.
Management of charge injection for safe stimulation in commercial stimulators today is performed using one variable (charge density) on an electrode-by-electrode basis. This approach is sufficient for present-day stimulation systems and electrode surface areas, because side-effects prohibit a clinician from practically reaching a tissue safety limit. In particular, a patient undergoing neurostimulation therapy would be expected to exhibit side effects well before cell trauma would occur. The onset of side-effects is primarily caused by the total charge per pulse, thereby naturally limiting the total charge per pulse (as well as the charge density per pulse) that can be applied to the patient. Due to the relatively large area, and resulting low charge density, of prior art electrodes, the charge density per pulse is also naturally limited by the side-effects experienced by the patient.
While managing charge injection for safe stimulation based on the charge density for each electrode may be acceptable for conventional neurostimulation systems, such charge injection management does not adhere to the Shannon model. For example, if electrical current at 450 μs and 4 mA is delivered to a single active electrode having a surface area of 0.06 cm2, the charge, charge density, and k-value are 1.8 μs, 30 ρC/cm2, and 1.73, respectively. If the amplitude of the electrical current is doubled to 8 mA, and the surface area of the electrode is doubled to 0.12 cm2, the charge-density remains the same (30 ρC/cm2), but the k-value increases substantially to 2.03. This example shows that a charge-density limit alone does not manage the k-value, and can result in breaches of a k-value threshold designed for one electrode.
Although conformance with the Shannon model may not be necessary when an electrode is relatively large, as the size of electrodes becomes smaller (e.g., the use of segmented electrodes is becoming prevalent in the context of DBS), thereby effectively increasing the charge density per pulse, it may be possible to cause cell trauma before the onset of side-effects. Therefore, an improved charge management solution is needed as new leads are developed with smaller electrodes and side-effects cannot be relied on to naturally manage adherence to the Shannon model.
It is possible that for a case of multiple active electrodes, an approach that relies on the Shannon model (or a surrogate parameter for k, such as charge-density or charge as a function of surface area), but which replaces the electrode surface area with a cumulative or effective contact surface area (e.g., could be a sum of active electrode surface areas or the sum of active electrode surface areas multiplied by a dispersion factor greater than 1 to get credit for the expanded spatial distribution) could be used. Such an approach, which is in essence a reduction of the problem to the Shannon model, seems reasonable for the case where a single electrical source is used and all active electrodes (of the same polarity) are at the same potential. However, use of independent electrical sources (e.g., like multiple independent current control (MICC) devices) can create distributions of currents that are not readily reduced to the Shannon model, and a new approach is needed.